1. Field of the Invention
The present invention relates to an image forming apparatus (e.g., a printer, a copier, a facsimile machine, or a multifunction peripheral) that has a plurality of image bearing members placed along an intermediate transfer portion or a recording material conveying portion and forms an image when the intermediate transfer portion or the recording material conveying portion is separated from part of the image bearing members. In particular, the present invention relates to control of a transfer voltage used in transferring a toner image on an image bearing member.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2005-062642 discloses an image forming apparatus that has a plurality of photosensitive drums placed along an intermediate transfer portion and that executes a color mode in which an image is formed when the intermediate transfer portion, which is a belt member, is in contact with all the photosensitive drums and a black monochrome mode in which an image is formed when the intermediate transfer portion is separated from part of the photosensitive drums. Another image forming apparatus having a similar configuration is disclosed in Japanese Patent Laid-Open No. 2004-117426. This image forming apparatus has a plurality of photosensitive drums placed along a recording material conveying portion and executes a color mode in which an image is formed when the recording material conveying portion is in contact with all the photosensitive drums and a black monochrome mode in which an image is formed when the recording material conveying portion is separated from part of the photosensitive drums. With the image forming apparatus disclosed in either of these patent documents, the longevity of photosensitive drums corresponding to magenta, cyan, and yellow toners, which are not used in forming a black image, can be increased by separation of the intermediate transfer portion or recording material conveying portion from these photosensitive drums during black-image formation.
The above-described image forming apparatus uses a technique for controlling a transfer voltage using the so-called active transfer voltage control (ATVC), which applies a test voltage to a transfer member and sets a transfer voltage at which a predetermined current is passed based on a relationship between voltage and current at that time (see, for example, Japanese Patent Laid-Open No. 5-006112).
For the image forming apparatus described in Japanese Patent Laid-Open No. 2005-062642, the contact state of the intermediate transfer portion and the transfer member when the color mode is executed differs from that when the black monochrome mode is executed. The difference in the contact state changes the impedance of a transfer portion when a black toner image is transferred to the intermediate transfer portion, so it is necessary to set a transfer voltage corresponding to the contact state in each mode. Accordingly, when the transfer voltage is set using the ATVC process described in Japanese Patent Laid-Open No. 5-006112, measuring a current in both modes is needed.
However, measuring a current in both modes requires setting the transfer voltage based on the ATVC process every time the mode is switched, thus resulting in a decrease in productivity.